The federal government has enacted the Transportation Recall Enhancement, Accountability, and Documentation (TREAD) Act. The TREAD Act proposes to require that certain passenger vehicles eventually be equipped with a tire pressure monitoring system (TPMS). There are two basic types of TPMSs: direct TPMSs and indirect TPMSs. A direct TPMS includes a tire pressure sensor in each tire. The sensors transmit pressure information to a receiver, which can be utilized to alert a driver of potential low tire pressure conditions. An indirect TPMS does not include tire pressure sensors. One type of indirect TPMS relies on the wheel rotational speed sensors in an anti-lock braking system (ABS) to detect and compare differences in the rotational speed of a vehicle's wheels. Those differences correlate to differences in tire pressure because decreases in tire pressure cause decreases in tire diameter that, in turn, cause increases in wheel rotational speed.
A direct TPMS and other types of systems are powered by an internal battery, which is not easily replaced. For example, since each tire pressure sensor is located within a tire, replacing the battery would require removing at least a portion of the tire from the wheel. Additionally, it is generally cost prohibitive to adequately secure a replaceable battery in a TPMS. Since the battery lifetime depends on power consumption of the sensor device, it is often desirable to detect movement of an object, such as a tire of a passenger vehicle. The detected movement of the tire thus can be employed to control operation of the tire pressure sensor in a way to conserve battery life.
Accordingly, various approaches have been developed to detect movement of tires implementing a TPMS. For example, one approach is to employ an electromechanical device configured to detect movement of a vehicle tire, such as whether the tire is rolling or the tire is stationary. One type of device is a roll sensor or accelerometer switch that is operative to change states and provide an indication of movement of the associated tire. As an example, the roll sensor may be formed of a switch having a mechanical portion that is movable commensurate with movement of a rolling vehicle tire so as to change states in response to movement of the vehicle tire.
Such roll sensors are often utilized in tire pressure monitoring systems to initiate a change in operating mode from a sleep mode when the tire is stationary and wake up and enter into a normal operating mode when the tire is moving. A general purpose of the roll sensor is to conserve battery power for an associated sensing system by reducing power consumption, such as by less frequent operation, when the vehicle is not in use. The addition of such a roll sensor in the TPMS, however, results in additional cost associated with the sensor in order to achieve the improved battery life. Another approach to increase the lifetime of a device, such as a TPMS is to utilize a longer lasting power source or battery. When such a battery is used, a roll sensor can be omitted, the pressure within the tire can be sensed and transmitted more frequently. Again, the longer lasting battery further increases the overall cost of each device.